A Fagersta economizer, made in Sweden, is a shell and tube type heat exchanger having vertical heat transfer tubes for recovering low-temperature waste heat from the exhaust gas of a heavy oil-fired boiler, in which the heat exchanger can recover not only sensible heat of the exhaust gas but also latent heat of condensation contained in the water vapor. Such a heat exchanger has, as a cleaning system, a structure enabling water to be sprayed onto the upper portion of the heat exchanger in order to clean the heat exchanger.
However, if combustion in the boiler is not efficient, water vapor, acid gases and so on within the exhaust gas can be condensed to heat transfer surfaces such that the heat transfer surfaces remain in a wetted state, thereby promoting the adhesion of the dust in the exhaust gas to the heat transfer surfaces. In addition, the dust has stickiness and a tendency to solidify as time passes. Therefore, if such a situation persists, there is a problem in that the cleaning effect achieved by spraying water may be poor, and hence, the heat transfer tubes may be clogged due to the adhesion of fouling materials thereon.
Furthermore, heat transfer tubes coated with Teflon are used in some heat exchangers. Because the surface of Teflon is smooth, the dust does not readily adhere and can be easily cleaned away using water. However, once the dust in the exhaust gas has adhered and solidified on the heat transfer surfaces, there is a problem in that it is difficult to completely clean the fouled heat transfer tubes with a simple water cleaning process.
As an approach to overcome such problems, the Korea Institute of Energy Research has recovered waste heat from exhaust gas using a construction in which a perforated plate is installed in a flow path for exhaust gas of a dryer and a bundle of heat transfer tubes coated with Teflon is placed above the perforated plate. Because sticky dust is contained in the exhaust gas of the dryer, the dust adheres to the heat transfer surface and the adhered dust is gradually solidified. In order to clean the heat exchanger for recovering waste heat from the exhaust gas of the dryer, the heat transfer surfaces have to be cleaned by supplying water on the perforated plate for a given time period and forming a water fluidized bed within the array of heat transfer tubes.
However, it has been found that there the ability to clean the heat transfer surfaces using the water fluidized bed is limited after the fouling materials adhering to the heat transfer surfaces have been solidified.
Thus, in the case of the heat transfer tubes to which the fouled materials are solidified, after stopping operating of the heat exchanger, workers directly insert a high pressure cleaner into the heat transfer tubes, and clean the heat transfer tubes by spraying water onto the wall surfaces thereof at high pressure.
However, in the case as mentioned above, there are problems in that the maintenance costs is increased, the heat exchanger and the related process have to be stopped, and thus the productivity of the entire process suffers.
In addition, in another method, a cleaning process comprises dropping shot balls down into the heat transfer tubes.
Unfortunately, such a method has a problem that when the shot balls are dropped down, the heat transfer surfaces become damaged by the fast downward speed of the shot balls, and the contact efficiency between the vertical heat transfer surfaces and the shot balls is very low.
Further more, the method as mentioned above has another problem in that an apparatus such as a bucket elevator has to be used to deliver the shot balls, but such an apparatus is bulky and may lower overall economic efficiency.